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| United States Patent |
5,160,152
|
|
Toraguchi
, et al.
|
November 3, 1992
|
Electrostatic chuck
Abstract
Herein provided is an electrostatic chuck whose surface is processed so as
to have projections and recesses, which has a simple structure and which
makes it possible to establish a uniform temperature distribution on a
wafer surface when the wafer is held on the processed surface thereof
through the use of an electrostatic attractive force. The uneven surface
configuration of the electrostatic chuck is designed so that the
proportion of the area occupied by the projected surface in the peripheral
portion, i.e., in the relatively outer region of the surface, is smaller
than that of the area occupied by the projected surface in the central
portion, i.e., in the relatively inner region of the surface of the
electrostatic chuck, in order to change the rate of heat transmission so
as to be larger in the central portion than in the peripheral portion
between the wafer and the electrostatic chuck. In this case, the height of
the projections is limited to the range of from 10 to 70 .mu.m so as to
perform effective control of the temperature distribution on the wafer by
such adjustment of the proportion of the area occupied by the projections.
| Inventors:
|
Toraguchi; Makoto (Kawasaki, JP);
Katagiri; Genichi (Kawasaki, JP);
Sakakibara; Yasushi (Kawasaki, JP)
|
| Assignee:
|
Fuji Electric Co., Ltd. (Kanagawa, JP)
|
| Appl. No.:
|
667749 |
| Filed:
|
March 11, 1991 |
Foreign Application Priority Data
| Mar 13, 1990[JP] | 2-61681 |
| Jan 21, 1991[JP] | 3-4811 |
| Current U.S. Class: |
279/128; 361/234 |
| Intern'l Class: |
B25B 011/00; B23Q 003/15 |
| Field of Search: |
279/1 M,128
269/8
361/234
|
References Cited
U.S. Patent Documents
| 4645218 | Feb., 1987 | Ooshio et al. | 279/1.
|
| 4692836 | Sep., 1987 | Suzuki | 361/234.
|
| 5055964 | Oct., 1991 | Logan et al. | 279/1.
|
| Foreign Patent Documents |
| 171011 | Feb., 1986 | EP.
| |
| 1-251735 | Jun., 1989 | JP.
| |
| 313954 | Dec., 1989 | JP | 269/8.
|
| 2106325 | Apr., 1983 | GB.
| |
| 2147459 | May., 1985 | GB.
| |
| 2154365 | Sep., 1985 | GB.
| |
Primary Examiner: Bishop; Steven C.
Attorney, Agent or Firm: Spencer, Frank & Schneider
Claims
What is claimed is:
1. An electrostatic chuck, comprising: a body having a substantially
circular surface with an area equal to that of a disk-shaped substrate to
be processed, said circular surface having a plurality of concentric
regions including at least an inner region and an outer region, each
region having a projected surface which comes in contact with the
disk-shaped substrate, at least said outer region having a recessed
surface which does not come in contact with the disk-shaped substrate, the
projected surface in the outer region occupying a proportion of area of
the outer region which is no more than 50% of the proportion of an area
occupied by the projection surface in said inner region.
2. The electrostatic chuck according to claim 1, wherein the proportion of
the area occupied by the projected surfaces is continuously reduced in a
direction from the inner region to the outer region.
3. The electrostatic chuck according to claim 1, wherein the projected the
recessed surfaces have a difference in level which ranges from 10 to 70
.mu.m.
4. The electrostatic chuck according to claim 2, wherein the projected and
recessed surfaces have a difference in level which ranges from 10 to 70
.mu.m.
5. The electrostatic chuck according to claim 1, wherein said inner region
has a diameter which ranges from 10 to 50% of the diameter of the circular
surface.
6. The electrostatic chuck according to claim 3, wherein said inner region
has a diameter which ranges from 10 to 50% of the diameter of the circular
surface.
7. The electrostatic chuck according to claim 4, wherein said inner region
has a diameter which ranges from 10 to 50% of the diameter of the circular
surface.
8. The electrostatic chuck according to claim 1, wherein the projected
surface in the inner region occupies an area which ranges from 25 to 100%
of a total area of said inner region.
9. The electrostatic chuck according to claim 3, wherein the projected
surface in the inner region occupies an area which ranges from 25 to 100%
of a total area of said inner region.
10. The electrostatic chuck according to claim 6, wherein the projected
surface in the inner region occupies an area which ranges from 25 to 100%
of a total area of said inner region.
11. The electrostatic chuck according to claim 7, wherein the projected
surface in the inner region occupies an area which ranges from 25 to 100%
of a total area of said inner region.
12. The electrostatic chuck according to claim 1, wherein the projected
surfaces comprise projections arranged at predetermined intervals.
13. The electrostatic chuck according to claim 2, wherein the projected
surfaces comprise projections arranged at predetermined intervals.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrostatic chuck for holding a wafer
during treatment such as forming a thin film on the wafer or etching the
same in a process for producing semiconductor devices according to the
plasma CVD method or the plasma etching process and in particular to an
electrostatic chuck which is capable of maintaining the temperature of the
wafer uniform throughout the processing.
2. Description of the Prior Art
Until now, in the plasma CVD apparatuses and the plasma etching
apparatuses, a wafer to be processed has been positioned on a
substrate-supporting base under the influence of gravity or mechanically
clamped to the base.
In these methods for positioning a wafer in the foregoing apparatuses, for
instance, a method for positioning a wafer under the influence of gravity,
the temperature of a wafer greatly increases in proportion to an increase
in processing energy and the patterns formed on the wafer in the
preceeding process are correspondingly damaged because of the low heat
transmission rate in a vacuum. Therefore, the wafer cannot be treated at a
high processing rate. On the other hand, in the clamping method, the
processing precision of a wafer is impaired in the vicinity of the
clamping tool. Therefore, the clamping of only a part of the periphery of
the wafer is permitted during the processing, the contact pressure between
the wafer and a base for cooling is reduced and accordingly the cooling
efficiency is almost the same as that for the method in which the wafer is
positioned under the influence of gravity. Moreover, the wafer causes
mechanical strain even if a gas serving as a heat transmission medium is
introduced between the wafer and the cooling base, and thus the uniform
distribution of the wafer temperature cannot be ensured.
Thus, there has been a desire for the development of low temperature
processing in order to solve the aforementioned problems and to finely and
uniformly process the wafer. Under such circumstance, an electrostatic
chuck which makes use of electrostatic attractive force has been developed
and already put into practical use.
However, the electrostatic chuck which has been put into practical use
likewise suffers from a problem of establishment of uniform temperature
distribution in a wafer. To solve this problem, for instance, Japanese
Patent Application Laid-Open No. 1-251735 discloses an electrostatic chuck
having a complicated structure designed so as to introduce a gas between a
wafer and the electrostatic chuck while adjusting the pressure of the gas.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an
electrostatic chuck having a simple structure, which makes it possible to
establish uniform temperature distribution in a wafer to be processed
throughout processing thereof.
The inventors of this invention have conducted various studies to solve the
foregoing problem associated with the conventional electrostatic chuck.
They have discoverd that the heat transmission between a wafer and the
electrostatic chuck, in a vacuum, is caused by a contact heat transmission
mechanism therebetween; that the surface temperature distribution of the
electrostatic chuck, observed when no wafer is positioned and processing
energy is supplied, is such that the temperature essentially decreases in
a concentric manner, i.e., from the center thereof towards the periphery
along the radial direction; and that it is effective for establishing a
uniform temperature distribution in a wafer to gradually decrease the
effective contact area between the wafer and the electrostatic chuck from
the center thereof along the radial direction.
According to the present invention, the foregoing object can effectively be
achieved by providing an electrostatic chuck for holding a wafer, which
makes use of an electrostatic attractive force and which is processed to
make the surface thereof uneven so that the electrostatic chuck partially
comes in contact with the wafer, wherein the surface of the electrostatic
chuck is made uneven such that the proportion of the area occupied by
projected portions in the peripheral portion, i.e., in the relatively
outer region of the surface is smaller than that of the area occupied by
projected portions in the central portion, i.e., in the relatively inner
region of the surface of the electrostatic chuck in order to change the
rate of heat transmission between the wafer and the electrostatic chuck.
In the present invention, the surface configuration of the electrostatic
chuck may be designed such that the proportion of the area occupied by the
projected portions is continuously reduced from the center of the surface
to the outermost peripheral portion thereof.
Preferred surface configurations of the electrostatic chuck of the present
invention include, for instance, those having the difference in level or
the height of the projected portion ranging from 10 to 70 .mu.m and the
proportion of the area occupied by the projected portion in the peripheral
region being not more than 50% of the proportion of the area occupied by
the projected portion in the central region; those including a central
region which corresponds to the region whose diameter is 10 to 50% of the
diameter of the electrostatic chuck and which has a high proportion of the
area occupied by projected portion; and those in which the proportion of
the area occupied by the projected portion in the central region ranges
from 25 to 100%.
When a cooling jacket maintained at a constant temperature is fitted to the
reverse face of a disklike electrostatic chuck by clamping at the
peripheral portion and energy is uniformly supplied to the surface of the
electrostatic chuck free of a wafer, part of the energy is directed
towards the water-cooling jacket as heat flow and the remaining part of
the energy outwardly flows in the radial direction. This is because an
electrostatic chuck in general has a structure in which an insulating
base, a plate-like electrode and an insulating layer are laminated in this
order and integrated together and the sides thereof serve as radiating
surfaces. The cross section of the heat flow becomes large in proportion
to the distance from the center of the chuck (i.e., the radius of the
electrostatic chuck), but the energy within the region enclosed by the
circle of this radius increases in proportion to the square of the radius.
The density of heat flux directed towards the periphery along the radial
direction correspondingly increases as the radius increases and the
insulating material constituting the electrostatic chuck has a low thermal
conductivity. This leads to the establishment of a radial temperature
distribution on the surface of the electrostatic chuck in which the
temperature gradient is small in the central region and gradually
increases as the distance from the center increases or towards the
periphery of the chuck. More specifically, the temperature of the central
region of the electrostatic chuck is high compared with that observed at
the periphery thereof due to the radiation through the side faces of the
chuck and the heat transmission from the periphery of the chuck to the
water-cooling jacket. When a wafer is put on the electrostatic chuck, the
energy uniformly supplied to the wafer is transmitted to the electrostatic
chuck through radiation and the same temperature distribution as that
observed when no wafer is fitted to the chuck is established within the
electrostatic chuck while assuming the presence of fine gaps between the
wafer and the electrostatic chuck. On the other hand, heat transmission
due to the radiation corresponds to that from the electrostatic chuck to
the water-cooling jacket observed when no wafer is put thereon and
likewise the same temperature distribution as that for the electrostatic
chuck free of a wafer would be established within the wafer. In this
temperature distribution, the overall temperature of the wafer is of
course higher than that of the electrostatic chuck. Further, the thermal
resistance of the wafer is large since the thermal conductivity of the
wafer is high, but the wafer is thin and, therefore, the temperature
gradient of the wafer in the radial direction is greater than that of the
electrostatic chuck in the radial direction. As a result, the temperature
difference between the wafer and the electrostatic chuck becomes great in
the central portion and is gradually reduced as the distance from the
center (i.e., radius) increases or towards the periphery. For this reason,
if the contact area between the wafer and the electrostatic chuck having a
high heat capacity in the central region is greater than that in the
peripheral region, the contact thermal resistance in the central region
becomes lower than that observed in the peripheral region and the rate of
temperature drop observed in the central region becomes greater than that
observed in the peripheral region. Accordingly, the temperature gradient
within the wafer becomes low and thus the temperature of the wafer can be
made uniform.
In the electrostatic chuck according to the present invention, the height
of the projected portion formed on the surface of the chuck including both
central and peripheral regions thereof is limited to the range of from 10
to 70 .mu.m. This results from the fact that the electrostatic chuck in
general has a structure in which plate-like electrodes are embedded in an
insulating material comprising porcelain and the surface thereof on which
a wafer is attracted is grained to a surface roughness on the order of
.mu.m and. Therefore, it is difficult to achieve processing precision on
the order of 10 .mu.m or less from a technical standpoint, while if the
height is more than 70 .mu.m, the wafer cannot always be attracted to and
held on the chuck with certainty.
The above and other objects, effects, features and advantages of the
present invention will become more apparent from the following description
of embodiments thereof taken in conjunction with the accompanying drawings
.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross sectional view showing the whole structure of an
apparatus for holding a wafer provided with the electrostatic chuck
according to the present invention;
FIG. 2A is a plan view showing the surface configuration of a first
embodiment of the electrostatic chuck according to the present invention;
FIG. 2B is an enlarged view of a part of the peripheral portion of the
electrostatic chuck shown in FIG. 2A;
FIG. 3A is a plan view showing the surface configuration of a second
embodiment of the electrostatic chuck according to the present invention;
FIG. 3B is an enlarged view of a part of the peripheral portion of the
electrostatic chuck shown in FIG. 3A;
FIG. 4A is a plan view showing the surface configuration of a third
embodiment of the electrostatic chuck according to the present invention;
FIG. 4B is an enlarged view of a part of the central portion of the
electrostatic chuck shown in FIG. 4A;
FIG. 4C is an enlarged view of a part of the peripheral portion of the
electrostatic chuck shown in FIG. 4A;
FIG. 5A is a plan view showing the surface configuration of a fourth
embodiment of the electrostatic chuck according to the present invention;
FIG. 5B is an enlarged view of a part of the central portion of the
electrostatic chuck shown in FIG. 5A;
FIG. 5C is an enlarged view of a part of the peripheral portion of the
electrostatic chuck shown in FIG. 5A;
FIG. 6A is a plan view showing the surface coinfiguration of a fifth
embodiment of the electrostatic chuck according to the present invention;
FIG. 6B is an enlarged view of a part of the central portion of the
electrostatic chuck shown in FIG. 6;
FIG. 6C is an enlarged view of a part of the peripheral portion of the
electrostatic chuck shown in FIG. 6A;
FIG. 7A is a plan view showing the surface configuration of a sixth
embodiment of the electrostatic chuck according to the present invention;
FIG. 7B is an enlarged view of a part of the central portion of the
electrostatic chuck shown in FIG. 7A;
FIG. 7C is an enlarged view of a part of the peripheral portion of the
electrostatic chuck shown in FIG. 7A;
FIG. 8A is a plan view showing the surface configuration of a first
comparative embodiment of an electrostatic chuck given for the purpose of
making comparison with the electrostatic chuck according to the present
invention;
FIG. 8B is an enlarged view of a part of the electrostatic chuck shown in
FIG. 8A;
FIG. 9 is a plan view showing the surface configuration of a second
comparative embodiment of an electrostatic chuck given for the purpose of
making comparison with the electrostatic chuck according to the present
invention;
FIG. 10A is a plan view showing the surface configuration of an embodiment
of the electrostatic chuck according to the present invention, the surface
configuration being designed with emphasis on the cooling properties;
FIG. 10B is an enlarged view of a part of the second region shown in FIG.
10A;
FIG. 10C is an enlarged view of a part of the third region shown in FIG.
10A; and
FIG. 10D is an enlarged view of a part of the fourth region shown in FIG.
10A.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the attached figures, FIG. 1 is a cross sectional view of
the whole apparatus for holding a wafer provided with the electrostatic
chuck according to the present invention. The electrostatic chuck 1 is
attached to a water-cooling jacket 2 through an insulating plate 5 by
fastening them with bolts 3 at the periphery thereof. The water-cooling
jacket 2 has a central portion formed into a cylindrical shape in which a
distributing wiring is provided and the wiring is connected to power
supply terminals 4 for applying an electric voltage for attraction to the
electrostatic chuck 1. Sealing means are disposed between the
electrostatic chuck and the insulating plate 5 and between the insulating
plate 5 and the water-cooling jacket 2. The electrostatic chuck is
provided with a pin 6 for releasing the attracted wafer.
In this embodiment, the surface configuration of the electrostatic chuck 1
comprises a first region or a central region 1A consisting of a projection
7A and a second region or a peripheral region 1B concentric with the first
region 1A, on which projections 7B are formed.
In FIG. 1, h defines a difference of level between a projection and an
adjacent recess and ranges from 10 to 70 .mu.m in this embodiment (in the
figure, the size of h is exaggerated for the purposes of illustration.
In FIGS. 2 to 7, there are shown embodiments 1 to 6 of the electrostatic
chuck according to the present invention and FIGS. 8 and 9 show
comparative embodiments given for the purpose of making comparison with
the embodiments of the present invention. In FIGS. 2 to 9, the diameter of
the electrostatic chuck is 180 mm.
In the electrostatic chuck 1 shown in FIG. 2, the central region 1A is in a
circular shape having a diameter of 40 mm and the whole of the central
region 1A comprises a projected surface portion 7A as shown in FIG. 2A. On
the other hand, projections 7B having a diameter of 0.57 mm as a dot
pattern are arranged on the peripheral region 1B of the electrostatic
chuck 1 at regular intervals (pitch=1.5 mm) as shown in FIG. 2B (not shown
in FIG. 2A). In this first embodiment, the diameter of the central region
1A is about 22% of the diameter of the electrostatic chuck 1. In addition,
the proportion of the area occupied by the projection 7A with respect to
the total area of the central region 1A as the first region is 100% while
the proportion of the area occupied by the projections 7B with respect to
the total area of the peripheral region 1B as the second region is 11%.
In the electrostatic chuck 1 shown in FIG. 3, the central region 1A is in a
circular shape having a diameter of 60 mm and the whole of the central
region 1A comprises a projected surface portion 7A as shown in FIG. 3A. On
the other hand, projections 7B having a diameter of 1.13 mm as a dot
pattern are arranged on the peripheral region 1B of the electrostatic
chuck 1 at regular intervals (pitch=2.0 mm) as shown in FIG. 3B (not shown
in FIG. 3A). In this second embodiment, the diameter of the central region
1A is about 33% of the diameter of the electrostatic chuck 1. In addition,
the proportion of the area occupied by the projection 7A with respect to
the total area of the central region 1A as the first region is 100% while
the proportion of the area occupied by the projections 7B with respect to
the total area of the peripheral region 1B as the second region is 25%.
In the electrostatic chuck 1 shown in FIG. 4, the central region 1A is in a
circular shape having a diameter of 40 mm as shown in FIG. 4A, recesses 7A
having a diameter of 1.0 mm as a dot pattern are formed on the central
region 1A at regular intervals (pitch=2.0 mm) and the remaining flat
surface serves as a projection as shown in FIG. 4B. On the other hand,
projections 7B having a diameter of 0.57 mm as a dot pattern are arranged
on the peripheral region 1B of the electrostatic chuck 1 at regular
intervals (pitch=1.5 mm) as shown in FIG. 4C (not shown in FIG. 4A). In
this third embodiment, the diameter of the central region 1A is about 22%
of the diameter of the electrostatic chuck 1. In addition, the proportion
of the area occupied by the projections 7A with respect to the total area
of the central region 1A as the first region is 80% while the proportion
of the area occupied by the projections 7B with respect to the total area
of the peripheral region 1B as the second region is 11%.
In the electrostatic chuck 1 shown in FIG. 5, the central region 1A is in a
circular shape having a diameter of 40 mm as shown in FIG. 5A, recesses 7A
having a diameter of 3.5 mm as a dot pattern are formed on the central
region 1A at regular intervals (pitch=5.0 mm) and the remaining flat
surface serves as a projection as shown in FIG. 5B. On the other hand,
projections 7B having a diameter of 0.57 mm as a dot pattern are arranged
on the peripheral region of the electrostatic chuck 1 at regualr intervals
(pitch =1.5 mm) as shown in FIG. 5C (not shown in FIG. 5A). In this fourth
embodiment, the diameter of the central region 1A is about 22% of the
diameter of the electrostatic chuck 1. In addition, the proportion of the
area occupied by the projections 7A with respect to the total area of the
central region 1A as the first region is 62% while the proportion of the
area occupied by the projections 7B with respect to the total area of the
peripheral region 1B as the second region is 11%.
In the electrostatic chuck 1 shown in FIG. 6, the central region 1A is in a
circular shape having a diameter of 60 mm as shown in FIG. 6A, recesses 7A
having a diameter of 1.0 mm as a dot pattern are arranged on the central
region 1A at regular intervals (pitch=2.0 mm) and the remaining flat
surface serves as a projection as shown in FIG. 6B. On the other hand,
projections 7B having a diameter of 1.13 mm as a dot pattern are arranged
on the peripheral region of the electrostatic chuck 1 at regular intervals
(pitch =2.0 mm) as shown in FIG. 6C (not shown in FIG. 6A). In this fifth
embodiment, the diameter of the central region 1A is about 33% of the
diameter of the electrostatic chuck 1. In addition, the proportion of the
area occupied by the projections 7A with respect to the total area of the
central region 1A as the first region is 80% while the proportion of the
area occupied by the projections 7B with respect to the total area of the
peripheral region 1B as the second region is 25%.
In the electrostatic chuck 1 shown in FIG. 7, the central region 1A is in a
circular shape having a diameter of 60 mm as shown in FIG. 7A, recesses 7A
having a diameter of 3.5 mm as a dot pattern are arranged on the central
region 1A at regular intervals (pitch=5.0 mm) and the remaining flat
surface serves as a projection as shown in FIG. 7B. On the other hand,
projections 7B having a diameter of 1.13 mm as a dot pattern are arranged
on the peripheral region of the electrostatic chuck 1 at regular intervals
(pitch =2.0 mm) as shown in FIG. 7C (not shown in FIG. 7A). In this sixth
embodiment, the diameter of the central region 1A is about 33% of the
diameter of the electrostatic chuck 1. In addition, the proportion of the
area occupied by the projections 7A with respect to the total area of the
central region 1A as the first region is 62% while the proportion of the
area occupied by the projections 7B with respect to the total area of the
peripheral region 1B as the second region is 25%.
On the other hand, in the electrostatic chuck 1 shown is FIG. 8 as a
comparative embodiment, projections having a diameter of 0.57 mm
distributed in the form of a dot pattern are formed, at regular intervals
(pitch=1.5 mm) throughout the entire surface of the electrostatic chuck 1
including both central and peripheral regions as shown in FIG. 8B (not
shown in FIG. 8A).
In another comparative embodiment of an electrostatic chuck 1 shown in FIG.
9, no recess is formed on the entire surface i.e., including both central
and peripheral regions, which is hence completely even.
Using the electrostatic chuck 1 according to the embodiments shown in FIGS.
2 to 7 and the comparative embodiments shown in FIGS. 8 and 9, the
temperature distributions on the wafer and the electrostatic chuck
observed when a uniform plasma energy was supplied thereto were
determined. The results thus obtained are summarized in the following
Table 1.
TABLE 1
__________________________________________________________________________
Sample
Surface Con-
Temp. of Electrostatic Chuck
Temp. of Wafer (.degree.C.)
No. figuration
Center (.degree.C.)
Periphery (.degree.C.)
Center
Periphery
.DELTA.T
__________________________________________________________________________
1 FIG. 2 86 31 253 224 29
2 FIG. 3 67 31 182 116 66
3 FIG. 4 108 75 305 268 37
4 FIG. 5 100 76 332 275 57
5 FIG. 6 88 75 226 158 68
6 FIG. 7 83 77 247 164 83
7 FIG. 8 40 34 447 194 253
8 FIG. 9 57 32 157 54 103
__________________________________________________________________________
.DELTA.T means the temperature difference between the central region and
the peripheral region of the wafer.
As seen from the results listed in Table 1, .DELTA.T of the wafer was more
than 100.degree. C. when the comparative electrostatic chucks shown in
FIGS. 8 and 9 were used, while when the electrostatic chucks according to
the present invention as shown in FIGS. 2 to 7 were employed, the
temperature difference .DELTA.T was substantially reduced and rather
uniform temperature distributions were established on the wafer surface.
In particular, the best effects were attained by the embodiment shown in
FIG. 2.
According to the wafer attraction-release test performed by the inventors,
the lowest wafer temperature (most excellent cooling properties) can be
achieved by the electrostatic chuck shown in FIG. 9, but the attractive
force thereof is greatly influenced by the conditions (surface
configurations) of the reverse face of the wafer. The remaining attractive
force during the disconnection of the wafer is markedly great.
Accordingly, stable attraction and release of the wafer cannot be ensured
and thus the electrostatic chuck of this type cannot be practically
acceptable.
Another embodiment which differs from the foregoing embodiments and is
designed with emphasis on the cooling properties is shown in FIG. 10. As
seen from FIG. 10A, the surface of this electrostatic chuck is divided
into four regions which are concentrically arranged. The central region 1A
is in a circular shape having a diameter D.sub.1 of 76 mm and all of the
central region 1A constitutes or serves as a projected portion. On the
other hand, the peripheral region 1B comprises three ring-like regions
1B.sub.1, 1B.sub.2 an 1B.sub.3. The region 1B.sub.1 which corresponds to a
region existing between the diameters D.sub.1 and D.sub.2 (76 mm to 106
mm) has projections having a diameter of 4.88 mm as a dot pattern arranged
at regular intervals (pitch=5 mm) as shown in FIG. 10B (not shown in FIG.
10A). The region 1B.sub.2 which corresponds to a region existing between
the diameters D.sub.2 and D.sub.3 (106 mm to 130 mm) has projections
having a diameter of 1.6 mm as a dot pattern arranged at regular intervals
(pitch=2 mm) as shown in FIG. 10C. Further, the region 1B.sub.3 which
corresponds to a region existing between the diameters D.sub.3 and D (130
mm to 180 mm) has projections having a diameter of 1.13 mm as a dot
pattern arranged at regular intervals (pitch=2 mm) as shown in FIG. 10D.
In this embodiment, the proportion of the area occupied by the projections
with respect to the total area of each region 1A, 1B.sub.1, 1B.sub.2 and
1B.sub.3 is 100%, 72%, 50% and 25%, respectively.
The results of another experimental test performed using the electrostatic
chucks shown in FIGS. 9 and 10 are summarized in the following Table 2.
TABLE 2
______________________________________
Temperature of Wafer (.degree.C.)
Sample No.
Surface Configuration
Center Periphery
.DELTA.T
______________________________________
1 FIG. 10 113 91 22
2 FIG. 9 108 79 29
______________________________________
As seen from the results listed in Table 2, the wafer temperature observed
for the electrostatic chuck shown in FIG. 10 is almost the same as that
observed for the electrostatic chuck shown in FIG. 9 and the wafer
temperature distribution established by the electrostatic chuck shown in
FIG. 10 is slightly superior to that achieved by the electrostatic chuck
shown in FIG. 9.
From the foregoing temperature characteristics and attraction-release
characteristics of the electrostatic chuck, it can be concluded that the
difference in level or the height of the projection preferably ranges from
10 to 70 .mu.m; that the maximum contact area (proportion of the area
occupied by the projection) in the central region preferably ranges from
25 to 100%; that the proportion of the contact area in the peripheral
region is preferably not more than 50% of the proportion of the contact
area of the central region; and that the surface of the chuck is
concentrically divided into two regions, the boundary of which is a circle
having a diameter preferably corresponding to 10 to 50% of the diameter of
the electrostatic chuck, or preferably into three or more regions and the
proportion of contact area is preferably changed continuously.
According to the present invention, the heat transmission rate is
controlled by utilizing an electrostatic chuck as a wafer holder to hold a
wafer without coming in contact with the wafer surface to be processed,
forcing the electrostatic chuck to cool and simultaneously changing the
contact area between the wafer and the electrostatic chuck in the radial
direction. Therefore, it is not necessary to adopt any complicated
structure and/or auxiliary means for controlling temperature. As a result,
a wafer holder of high reliability can be provided with a low cost of
equipment.
In addition, the electrostatic chuck of the present invention makes it
possible to substantially reduce the probability of contamination of
wafers which becomes a cause of various property-changes in the resulting
semiconductor microdevices since the electrostatic chuck holds the wafers
without coming in contact with the wafer surface to be processed.
It is a matter of course that the optimum temperature for processing a
wafer can easily be established by properly controlling the temperature of
the cooling medium used for compulsorily cooling the electrostatic chuck
although any specific embodiment is not given in the foregoing
description.
The invention has been described in detail with respect to preferred
embodiments, and it will now be apparent from the foregoing to those
skilled in the art that changes and modifications may be made without
departing from the invention in its broader aspects, and it is the
invention, therefore, in the appended claims to cover all such changes and
modifications as fall within the true spirit of the invention.
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